Charged particle extracting magnet for an accelerator



y 19,67 G. R. LAMBERTSON 332 3 CHARGEDPARTICLE EXTRACTING MAGNET FOR AN ACCELERATOR Filed July 13, 1965 B 2% x (\I al Q #8 a, 98 M INVENTOR GLEN R. LAMBERTSON ATTORNEY. v

United States Patent 3,323,088 CHARGED PARTICLE EXTRACTING MAGNET FOR AN ACCELERATOR Glen R. Lambertson, Oakland, Calif., assignor to the United States of America as represented by the United States Atomic Energy Commission Filed July 13, 1965, Ser. No. 471,775 5 Claims. (Cl. 335-210) ABSTRACT OF THE DISCLOSURE This invention is a magnet particularly designed for extracting a charged particle beam from the beam orbit of a high energy particle accelerator such as a synchrotron. A long channel formed from a magnetic material has a longitudinal septum portion disposed immediately adjacent the beam orbit of the accelerator. A magnetic field is produced in the channel transverse to the septum. Beam particles directed into the channel across the thin septum are removed from the accelerator beam orbit and directed at external targets.

This invention relates generally .to apparatus for accelerating charged particles to high energy in a closed orbit and more particularly to means for extracting accelerated particles from the closed orbit of a particle accelerator. The invention described herein was made in the course of, or under, Contract W7405eng-48 with the Atomic Energy Commission.

In a charged particle accelerator such as a synchrotron, particles are circulated for many cycles around a closed orbit, additional energy being imparted to the particles in the beam over successive transits of the orbit. After the desired energy has been attained, the charged particles are typically utilized to bombard a target. While it is possible to rapidly insert a target into the path of the particle beam in an accelerator, it is frequently desirable that the charged particle beam be extracted out of the synchrotron orbit so that the bombardment of the target may be accomplished at a place remote from the accelerator where there is more room for the large amount of apparatus associated with a target. Also, the extracted particle beam can be alternately shifted to various target areas so that one target can be prepared while another is being bombarded. Owing to these and many other advantages of beam extraction, nearly all large accelerators utilize extracted beams.

The present invention is a new type of beam extractor specifically designed for use with a pro-ton accelerating synchrotron having very high energy but is also suitable for use with other types of accelerators. The usual beam extractor for high energy par-ticles utilizes a channel into which a portion of the beam passes. A magnetic field is provided within the channel to deflect the beam away from the accelerator orbit and into steering magnets for directing the beam toward external targets. Ideally, the magnetic field in the channel will be strictly confined to the channel and will not deflect any of the beam particles in the adjacent synchrotron orbit. Such a channel is generally, although not necessarily, disposed at the outer side of the beam orbit, the radius of the beam being enlarged when the beam is to be extracted so that the beam passes into the channel. In a high energy accelerator the beam radius can only be enlarged by a small increment for each transit around the accelerator. Thus the wall of the channel nearest the beam orbit, termed the septum, must be as thin as possible to avoid intercepting any substantial portion of the particles. In the usual beam extractor, the septum wall is a current conductor which produces a magnetic field within the channel to deflect the beam away from the accelerator beam orbit. Unfortunately, the septum wall must have substantial thickness in order to conduct the necessary current without excessive heating but the leading edge of such septum then intercepts an excessive proportion of the beam particles, which causes further heating problems in the septum and, of course, the magnitude of the extracted beam is greatly reduced. Furthermore, beam particles striking the septum cause a high radioactivity level, both at the septum and for a considerable distance along the beam orbit.

The present invention avoids many of the above described problems. Instead of deflecting the beam in the channel outwardly from the synchrotron, in the present invention the beam is deflected transverse to the orbital plane, that is, if the synchrotron orbit lies in a horizontal plane, then the present invention deflects the beam vertically either upward or downward. By utilizing such deflection system, it is not necessary for the septum to carry current, thus the septum can be made very thin, and a relatively small portion of the beam particles strike the septum edge.

It is an object of the present invention to provide a means for extracting a particle beam from an accelerator in which the beam is separated from the plane of the beam orbit as well as being diverged therefrom.

It is an object of the present invention to improve effi-ciency of the beam extraction system of a charged particle accelerator and reduce induced radioactivity incident to beam extraction.

It is another object of the present invention to reduce 'the degree of cooling required in the septum wall of a beam extracting channel.

It is another object of the present invention to provide a septum wall for an accelerator beam extractor of greatly reduced thickness as compared to septum walls in previous beam extractors.

It is another object of the present invention to provide a particle accelerator beam extractor in which it is unnecessary for the septum wall to conduct current.

The invention, together with further objects and advant-ages thereof will be best understood by reference to the accompanying drawing in which:

FIGURE 1 is a schematic view of a portion of a synchrotron utilizing the present invention,

FIGURE 2 is a perspective view of a septum magnet indicated schematically in FIGURE 1, and

FIGURE 3 is a cross-sectional view of the septum magnet taken at line 3-3 in FIGURE 2.

Referring now to FIGURE 1, there is shown a segment 11 of a conventional synchrotron having an annular orbit 12 for an ion beam 9. In FIGURE 1, the vacuum system, magnet physical support structure, radio frequency system, and shielding, for which suitable structure is well known in the art, are not shown so that the positioning of the various components of the present invention may be more clearly indicated. The beam 9 circulates around the synchrotron 11 in a clockwise direction, passing through a plurality of beam guiding magnets 13 which hold the beam in the annular orbit 12. The magnet-s 13 are disposed in alternate focussing and defocussing pairs to provide alternating gradient focussing. When the ion beam 9 is to be extracted, a perturbation magnet 14 is activated, causing the beam to be shifited inwardly from the normal orbital path and all or a portion of the beam enters a first septum magnet 16. Such beam portion is further deflected away from the normal beam orbit by the first septum magnet 16. The beam 9 subsequently passes through a second septum magnet 17 where it is still fur ther deflected and is finally guided away from the synchrotron by final extraction septum magnets 18, the beam being directed along an external beam path 19 for bombardment of various targets.

The general structure of the first septum magnet 16 is shown in FIGURE 2 while certain details and the internal structure thereof are shown in FIGURE 3. Referring now to FIGURE 2 in particuluar, there is shown a portion of a long vacuum tube 21 which encloses the normal beam orbit 12 around the synchrotron, the general shape of the beam 9 being indicated by dashed lines. The septum magnet 16 is structurally supported by and affixed to a cylindrical strongback 22 to which mounting brackets 23 are bolted to support the magnet.

A U-shaped magnet core 31 of high magnetic permeability is shown in FIGURE 3, two pairs 32 and 33 of liquid-cooled electrical conductors being disposed adjacent the inside of each of the legs of the core, leaving a beam deflecting space 34 between the conductor pairs. A septum element 36 is disposed across the legs of the U-magnet core 31, in cross-section the septum core being thick adjacent the legs at the core and tapering towards the region midway therebetween to form a thin central septum 37. Preferably the septum core 36 is made of pure iron to avoid the changes in crystalline structure which could occur in alloys if the septum were overheated by the beam 9. Cooling fluids are circulated through tubes affixed to upper and lower surfaces of the U-core 3-1 and septum element 36.

In operation, current from a direct current power supply 30 is passed in one direction through conductor pair 33 and in the opposite direction through conductor pair 32, creating a magnetic field directed substantially across the space 34 from the thin septum 37 to the core 31. Essentially all the magnetic field is confined within magnet 16. When the beam 9 is caused to enter the region inside the thin septum 37 into space 34, the beam particles are deflected downward toward the lower conductor pair 32, that is, transverse to the plane in which the synchrotron beam orbit lies. The septum 37 may typically have a thickness in the order of 0.4 millimeter, thus only a small portion of the beam 9, which typically has a width of one centimeter, is intercepted even if the center of the beam should strike the septum.

The beam 9 or the intercepted portion thereof is passed along the entire length of the magnet 16, emerging substantially below the plane of the beam orbit 12. As shown in FIGURE 1, the deflected beam is then following a path below and essentially parallel to the beam orbit 12. Thus it will usually be necessary to again deflect the beam in the second septum magnet 17, directing the beam under and radially outwardly from the orbit 12, the beam being extracted by final extraction magnets 18 into the external beam path 19. It is, of course, optional whether the beam is deflected over or under the orbit 12 this being determined by the direction of the magnetic field in space 34. In some instances it may be desirable to retract the magnet 14- from the orbit 12 during the beam acceleration period. The magnet 16 is then rapidly advanced toward the beam orbit 12 prior to beam extraction, suitable mechanisms for this purpose being known to the art.

While the invention has been disclosed with respect to a particular embodiment, it will be apparent to those skilled in the art that numerous variations and modifications may be made within the spirit and scope of the invention and it is not intended to limit the invention except as defined in the following claims.

What is claimed is:

1. In a septum magnet for deflecting a beam of charged particles from the beam orbit of an accelerator, the combination comprising means forming a channel having a longitudinal beam passageway therethrough, said channel being made of a material of high magnetic permeability, said channel having a thin septum wall section adjacent said beam orbit and disposed transversely with respect to the plane of said orbit, and means providing a magnetic field within said channel which is transverse to said septum whereby said beam particles in said channel are deflected transversely with respect to said plane.

2. A septum magnet as described in claim 1, wherein said means providing a magnetic field is comprised of at least a pair of electrical conductors disposed within said channel on opposite sides of said passageway.

3. In a septum magnet for a charged particle accelerator beam extraction system, the combination comprising a hollow channel of magnetic material having a generally rectangular cross-sectional configuration with first, second, third and fourth sides, said channel having a thin longitudinal septum portion along the center of said first side of said channel, first and second current conductors disposed within said channel adjacent said second and fourth facing sides of said channel respectively, said first and second conductors being disposed adjacent said first wall on each side of said septum.

4. In a septum magnet for a charged particle accelerator beam extractor, the combination comprising a channel formed of material of high magnetic permeability and having a first wall therein for disposition adjacent the beam orbit of said accelerator, said first wall having a thin septum portion along the center thereof, said first wall being of increased thickness on each side of said septum portion, at least a pair of current conductors disposed adjacent said thick portions of said wall on opposite sides of said septum portion and extending along said wall within said channel.

5. A septum magnet as described in claim 4, wherein said magnetic material in said first wall is essentially pure iron.

References Cited UNITED STATES PATENTS 3,128,405 4/1964- Lam-bertson 32S235 X 

1. IN A SEPTUM MAGNET FOR DEFLECTING A BEAM OF CHARGED PARTICLES FROM THE BEAM ORBIT OF AN ACCELERATOR, THE COMBINATION COMPRISING MEANS FORMING A CHANNEL HAVING A LONGITUDINAL BEAM PASSAGEWAY THERETHROUGH, SAID CHANNEL BEING MADE OF A MATERIAL OF HIGH MAGNETIC PERMEABILITY, SAID CHANNEL HAVING A THIN SEPTUM WALL SECTION ADJACENT SAID BEAM ORBIT AND DISPOSED TRANSVERSELY WITH RESPECT TO THE PLANE OF SAID ORBIT, AND MEANS PROVIDING A MAGNETIC FIELD WITHIN SAID CHANNEL WHICH IS TRANSVERSE TO SAID SEPTUM WHEREBY SAID BEAM PATICLES IN SAID CHANNEL ARE DEFLECTED TRANSVERSELY WITH RESPECT TO SAID PLANE. 